Thermophotovoltaic (TPV) generators are combined with residential gas furnaces in a number of ways. One method is to treat the TPV unit as a first heat stage. This method requires that the single stage furnace be sized to handle the coldest days. To ensure that the furnace will be able to maintain the space temperature, the furnace is typically oversized. A TPV first stage that is 20% of the main furnace output is sufficient for 50% or more of the heating hours, and comfort is improved since the heat output more closely matches the space heat load. The furnace blower operates at a low rpm that matches the pressure output of the TPV generator. This operation is done with a PWM DC motor, a device that is already available in the HVAC industry. Two stage heat thermostats are commercially available. The TPV controls ensure safety of the hybrid system. The principal feature is enhanced comfort. Temperature swings are reduced at low loads, and continuous air movement also provides filtration. The TPV add-on serves as a backup furnace in the event there is a loss of power. The TPV unit is not able to satisfy the second stage load, but it at least maintains a significant space temperature relative to outdoor conditions. With the addition of electrical storage, full furnace output may be provided for a duration determined only by the amount of storage provided. The backup furnace may be done without using the TPV unit as a first stage, but the added value of a first stage operation makes the most sense.
The small, portable, cylindrical TPV generator provides electricity. A significant market exists for self-powered furnaces. A forced air self-powered furnace requires a 200 W DC blower and a 200 W electric generator to operate the blower. An infrared (IR) emitter and TPV cells provide electric power but require a new redesign for each different model furnace. Furthermore, the furnace requires 200 W of electricity, and a potential customer would have to purchase a new TPV powered furnace.